Resettable circuit breaker for use in ground fault circuit interrupters and the like

ABSTRACT

A resettable circuit breaker has a pair of normally closed contacts through which a load is connected to a power line and means for opening the contacts in response to a predetermined change in electrical current flowing to and from the power line. One of the contacts is mounted on a rocking member which is maintained between a pivot on the base of the apparatus and the armature of a latching solenoid responsive to the predetermined current change by a spring assembly which causes the rocking member to rotate about the pivot means thereby opening and closing the contacts depending on the position of the armature.

This is a continuation of application Ser. No. 532,838 filed Jun. 4,1990, now abandoned.

Ser. No. 532,838 is a continuation of Ser. No. 404,802, filed Sep. 8,1989, and now abandoned.

Ser. No. 404,802 is a continuation of Ser. No. 219,413, filed Jul. 14,1988, and now abandoned.

Ser. No. 219,413 is a continuation of application Ser. No. 126,535,filed Nov. 30, 1987 and now abandoned, which, in turn, is a continuationof Ser. No. 771,439, filed Aug. 30, 1985, and now abandoned.

Ser. No. 771,439 is a continuation of Ser. No. 540,078, filed Oct. 7,1983, and now U.S. Pat. No. 4,568,997, issued Feb. 4, 1986.

Ser. No. 540,078 is a continuation of Ser. No. 940,224, filed Sep. 7,1978, and now U.S. Pat. No. 4,412,193, issued Oct. 25, 1983.

BACKGROUND OF THE INVENTION

The danger of electrocution or of serious shock to an individual comingin contact with an appliance surface having an electrical potentialdiffering from ground potential when the individual was also touching agrounded surface created the need for ground fault circuit interrupters.Ground fault circuit interrupters cut off the supply of electric currentto a "hot" surface immediately upon flow of electric current to groundthrough other than the normal return path of the circuit in which theappliance is connected. Early ground fault circuit interrupterscontinuously measured electrical potential to ground and cut off powerto the monitored appliance in response to a predetermined change inpotential. These early devices were inherently faulty in that it wasdifficult to obtain a true ground point from which to measure electricalpotential and variations in ground impedance adversely affected theiroperation.

Improved ground fault circuit interrupters were later developed. Thesemonitored the difference between the electrical currents flowing intoand out of the power line to which they were connected. Ideally, thesupply and return currents are equal and their difference has amagnitude of zero. Should an external electrical path from the phase orhot side of the supply line to ground be completed, the current at thegrounded neutral terminal of the supply line decreases relative to thecurrent at the phase terminal of the line. The improved ground faultcircuit interrupters employed a differential transformer which generateda pulse upon the occurrence of an imbalance between the phase andneutral currents and this pulse was applied directly to a solenoidcausing the solenoid to actuate thereby opening a set of contactsbetween the power line and appliance or load. In order to function suchground fault circuit interrupters required very large transformers, toobulky to permit mounting of the ground fault circuit interrupter in astandard wall receptacle and highly sensitive solenoids which like thetransformers were very expensive.

Prior art ground fault circuit interrupters were also prone to trippingdue to noise on the power line which created momentary imbalance betweenthe phase and neutral currents. Other problems associated with prior artground fault circuit interrupters included an inability to cut off powerif inadvertently wired into a circuit in reverse, that is, with the loadterminals connected to the power line and power line terminals connectedto the load. In these devices when the ground fault circuit interrupterwas connected in reverse, not only was the power continuously suppliedto the load irrespective of actuation of the interrupter, but also incases where the opening of the contacts was accomplished by anelectromagnet, current would continue to flow in the electromagnet afterthe interrupter was actuated and the coil, normally designed only forintermittent duty, would burn out.

Another problem found in the prior art ground fault circuit interrupterswas attributable to the use of contact reset mechanisms which permitteda user to open and reset the contacts at will. To insure utmostreliability in a ground fault circuit interrupter, it is desirable thatthe contact tripping mechanism be subject to limited duty, i.e., onlywhen a ground fault occurs. Prior art ground fault circuit interrupterspermitted a user to open the contacts manually in the absence of aground fault thereby offering the temptation for the user to use thereset mechanism as a switch for turning an appliance on or off from thepower outlet. Such use often resulted in the deterioration of thecontacts and compromised the reliability of the ground fault circuitinterrupter.

Summary of the Invention

The previously mentioned problems associated with ground fault circuitinterrupters of the prior art are overcome by the instant inventionwhich includes a resettable circuit breaker having normally closed maincontacts, means for connecting a load to one side of said contacts,means for connecting a power line to the other of said contacts, meansfor interrupting the connection between the load and power line byopening the contacts in response to a predetermined change in thecurrent in the power line and means for resetting (closing) the contactsin the absence of the predetermined change in current. Ground faultcircuit interrupters embodying the invention may include a differentialtransformer for sensing a current imbalance between the phase andneutral power lines to which the ground fault circuit interrupter isconnected, sensor circuitry for providing an output signal when thecurrent imbalance exists for a predetermined time at a predeterminedmagnitude, and a SCR responsive to the output of the sensor circuitry.The SCR is in circuit with the coil of a latching relay according to theinvention to cause the contacts to open when the SCR conducts inresponse to a current imbalance. Auxiliary contacts responsive to themain contacts are provided for preventing application of current to therelay coil after occurrence of a ground fault irrespective of whetherthe ground fault circuit interrupter is properly wired in circuit.

The relay includes an armature slidably and pivotally mounted within aframe and connected to a coil spring. A separate spring assemblyconnects the armature to a rocking member on which one of the contactsis mounted, the rocking member being pivotal with respect to the chassisof the ground fault circuit interrupter to rotate in one direction toopen the contacts when the armature of the relay is unlatched uponenergization of the coil and to rotate in the opposite direction toclose the contacts when the armature of the relay is depressed and heldin a latched position under the tension of the coil spring.

It is, therefore, an object of the invention to provide a resettablecircuit breaker for use in a ground fault circuit interrupter whereincurrent flow is immediately interrupted upon completion of a circuitbetween ground and either the phase or neutral connections to a load.

Another object of the invention is to provide a ground fault circuitinterrupter which can be conveniently packaged for installation in astandard wall outlet receptacle.

Still another object of the invention is to provide a ground faultcircuit interrupter which actuates in response to the magnitude ofcurrent flow in an unwanted external circuit and the time during whichexternal current flow occurs.

A further object of the invention is to provide a ground fault circuitinterrupter immune to coil burn-out when installed in reverse.

A still further object of the invention is to provide a ground faultcircuit interrupter with a compact yet positive resettable latchingrelay for permitting current flow from a power line to a load only inthe absence of ground faults.

An additional object of the invention is to provide a tease-proof groundfault circuit interrupter which, once reset into a conducting condition,cannot be rendered non-conducting absent the occurrence of a groundfault.

Other and further objects of the invention will be apparent from thefollowing drawings and description of a preferred embodiment of theinvention in which like reference numerals are used to indicate likeparts in the various views.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electrical circuit schematic diagram of the apparatus ofthe invention;

FIG. 2 is a front view of the ground fault circuit interrupter of theinvention;

FIG. 3 is a sectional view of the apparatus of the invention, showingthe apparatus of the invention in one state of operation in its intendedenvironment;

FIG. 4 is a sectional view similar to the view of FIG. 3 of a part ofthe apparatus during transition to another state of operation in itsintended environment; and

FIG. 5 is a view similar to FIG. 3 showing the apparatus after thecompletion of transition to its other state.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the circuit schematic wiring diagram of FIG. 1 of thedrawings, there are shown input terminals 1 and 3 which are respectivelyconnected to the phase and neutral wires of a 60 cycle a.c. power lineas, for example, an electrical supply circuit leading from a fuse box ina residence or commercial building. The neutral terminal 3 is connectedby a lead 5 to the outer conductor of a coaxial cable 7 at the line endof the cable. The outer conductor of the cable 7 can be a rigidconducting hollow cylindrical conduit. The center conductor or phaselead 9 of the cable 7 is connected to the phase terminal 1 at its lineend and to a contact 11 of a relay actuated switch 13 at its load end. Aconductor 15 is connected at its line end to the load end of the outerconductor of the cable 7 and at its load end to a contact 17 of a switch19 which is structurally combined with the switch 13 for joint actuationwith it as will later be described.

Load terminals 21 and 23 are respectively connected to a load phase wire25 and a load neutral wire 27. The load phase wire 25 and load neutralwire 27 are respectively connected to contact 31 of the switch 13 andcontact 29 of the switch 19. The switches 13 and 19 are normally closedproviding electrical continuity between input terminal 1 and outputterminal 21 and between input terminal 3 and output terminal 23. Uponoccurrence of a ground fault, the switches 13 and 19 are openedinterrupting the flow of current between contacts 17 and 29 and betweencontacts 11 and 13 as will subsequently be explained.

A differential transformer 33 has a core circumscribing the neutral wire5 and hot wire 9 which serve as the transformer primaries and asecondary winding having output leads 35 and 37 which are connected tothe input of a ground fault circuit interrupter sensing circuit 39.Capacitor 44 provides the necessary feedback for the integrator portionof the circuit 39. The circuit 39 as shown in FIG. 1 can be a MotorolaMC3426 which is commercially available. National Semiconductor Inc.(NSI) makes and sells an integrated circuit LM1850 functionally similarto the Motorola circuit which can also be purchased and used in acircuit similar to that of FIG. 5 but modified slightly in accordancewith known requirements of the NSI circuit.

Noise suppression for the circuit 39 is provided by resistor 42 andcapacitor 46. Capacitor 48 provides filtration for the circuit 39. Theoutput of the circuit 39 is applied via lead 41 to the gating stage 43of a silicon controlled rectifier (SCR) 45 having an anode 47 and acathode 49. Capacitor 51 connected in parallel with the SCR 45 providesnoise suppression for the SCR 45.

A difference in the respective currents flowing through the primarywindings of the transformer 33 induces a voltage pulse in the secondarywinding which appears across leads 35 and 37 and is applied to thecircuit 39 where it is integrated, and if the pulse is of sufficientmagnitude and duration, a pulse is generated at the output of thecircuit 39 on line 41 and applied to the gating stage 43 to render theSCR 45 conducting.

The coil 61 of an electromagnet is connected between the anode 47 of theSCR 45 and an auxiliary contact 63 which is connectable to the contact31 by a plunger actuated switch 65. The switch 65 is mechanicallycoupled with switches 13 and 19 in a manner to be described so thatswitches 13, 19 and 65 all open when coil 61 is energized and all closewhen the coil 61 is de-energized and the contacts are reset as willlater be described. When the SCR 45 is rendered conducting in responseto a current imbalance in the leads 5 and 9, it causes switches 19, 13and 65 respectively connected between contacts 17 and 29, contacts 11and 31, and contacts 31 and 63 to open and remain open, irrespective ofwhether the coil 61 remains energized or is de-energized, until theswitches 19, 13 and 65 are manually reset.

A normally open plunger actuated test switch 71 is provided between acontact 73 which is connected by line 75 to the load phase wire 27 and acontact 77 which is connected through a resistor 79 to the line neutrallead 5. Closing of the switch 71 provides a path for current to flowfrom the phase wire 25 to ground through the neutral lead 5 bypassingthe transformer 33. A current imbalance then exists between the currentflowing in the outer conductor 7 and the current flowing in the innerconductor 9. This current imbalance is sensed by the differentialtransformer 33 which causes the contacts associated with switches 13, 19and 65 to open as previously described.

The series combination of a light-emitting diode 81, a resistor 83 anddiode 85 is connected in parallel with the load terminals 21 and 23 andis lit when the switches 13 and 19 are closed and power is applied tothe line terminals 1 and 3 of the ground fault circuit interrupter. Thelit LED 81 provides a visual indication that the circuit in which theground fault circuit interrupter is installed is operating normally,i.e., without any ground fault. Upon occurrence of a ground fault whichcauses the switches 13, 19 and 65 to open power to the LED 81 is cut offand the light is extinguished. An extinguished LED provides a visualindication that a fault exists in the circuit.

The LED can also be used to test for reverse installation of the groundfault circuit interrupter. Should the load terminals 21 and 23 beinadvertently connected to the power line and the line leads 1 and 3mistaken for the load leads, the LED 81 will remain lit at all times,even after the test button 71 is actuated since power will becontinuously applied to the terminals 21 and 23 across thelight-emitting diode circuit.

The disclosed circuit, by employing auxiliary switch 65 between thecontacts 31 and 63 to selectively connect the relay coil 61 to the loadphase wire 25, prevents burn-out of the coil 61 upon an inadvertentreversed connection of the ground fault circuit interrupter. If the loadside of the coil 61 were permanently connected to load phase wire 25,inadvertent reverse connection of the ground fault circuit interrupter,whereby the load terminals 21 and 23 would be connected directly to thepower line, would result in a continuous flow of current through thecoil 61, even after the contact switches 13 and 19 were tripped. Thiswould likely result in burn-out of the coil 61 which is intended onlyfor intermittent duty. Under the arrangement shown in FIG. 1, even ifpower is supplied directly to the load terminals 21 and 23, once theinterrupter trips opening switches 13 and 19, mechanically coupledswitch 65 also opens thereby preventing current flow to the coil 61 and,hence, avoiding its burning out.

A power transformer 91 axially displaced from the transformer 33 alsocircumscribes the outer and inner conductors 7 and 9 for detecting thecompletion of a neutral to ground circuit at the load. Such anoccurrence can result in current flow from the ground to which the lineside of the neutral wire 5 is connected, to the line side of the neutral5, to the load side of the neutral 5, to the unwanted external ground,and back to the original ground to which the line side of the neutralwire 5 is connected. The primary winding of transformer 91 is connectedbetween load phase wire contact 31 through auxiliary switch 65, and loadneutral contact 29 through switch 19. The secondary winding of thetransformer 91 is connected in series with a capacitor 76 and a metaloxide varistor 74 is connected in parallel with the load for voltagetransient suppression. When a ground to neutral circuit is completed,current flows in it induced by the inductively coupled transformer 91.The flow of current causes an imbalance between the currents in theneutral and phase conductors adjacent transformer 33 and is sensed inthe same manner as a ground fault as previously described.

The mechanical operation of a ground fault circuit interrupter accordingto the invention will now be described with reference to FIGS. 2, 3, 4and 5.

Referring to FIG. 2, there is shown the face or outlet side of anelectrical outlet or receptacle adapted to receive two a.c. groundingplugs each of which is connected to an appliance or load. The outletalso has a reset button 101 which, when depressed, closes themechanically coupled switches 13, 19 and 65. The test button 71 is alsoaccessible from the face of the outlet for causing an intentionalgrouond fault for testing the interrupter as previously explained. Thelight-emitting diode 81 can be seen on the face of the outlet panel andwill normally be lit when the interrupter is in a reset condition, thatis, when the reset button 101 has been depressed and there has been nosubsequent ground fault, and will be extinguished when and after aground fault occurs. To test the unit when it is in a reset condition,the test button 71 is pressed at which time, if the unit is properlyinstalled, the light-emitting diode 81 should change from a lit to anextinguished state. Pressing the reset button 101 should restore theterminated power to the outlet and cause the LED 81 to be lit again.

Referring now to FIG. 3, the ground fault circuit interrupter of theinvention is contained in a lower housing 103 and an upper housing 105which mates with the lower housing 103 to form a substantially enclosedcasing for the mechanical and electronic components of the ground faultcircuit interrupter.

The reset button 101 has an upper surface 106 which is adapted to haveapplied to it finger pressure to close the switches 13, 19 and 65. Ametal frame 107 has a floor 109 and two vertical spaced parallel sideplanar members 111 extending transversely from opposite edges of thefloor 109. The side planar members 111 of the frame 107 each have asymmetric slot 113 with the uppermost edge of each of the slots 113having an inclined region 115 and a horizontal region 117 laterallydisplaced from the inclined region 115. The laterally displaced edgeregions 115 and 117 of the slot 113 are separated by a shoulder 119. Theslot 113 tapers along the downward inclination of the edge region 115toward the shoulder 119 and then abruptly widens at the shoulder 119where the edge region 117 of the slot 113 is vertically displaced fromthe edge region 115.

An armature 116 is pivotally connected at one of its ends to the resetbutton or plunger 101 and has the other of its ends slidably disposed ineach slot 113 of the frame 107. The armature 116 is substantiallyrectangular in shape with four ears projecting outwardly in paralleldirections from each of the four corners of the armature 116. Oppositeears 121 are rotatably disposed in circular apertures 123 formed in theside walls of the reset button 101 and opposite ears 125 are slidablydisposed in the slots 113 of the frame 107. The edge of the armature 116between the ears 125 is provided with a hook-like member which may beformed by notching and bending a portion of the armature material toreceive one end 127 of a coil spring 129. The other end 131 of the coilspring 129 is hooked over a notched member 133 which is formed of one oftwo inward-directed frame portions 135 which are connected to the framevertical members 107 and which are in parallel spaced relationship tothe floor 109 of the frame 107. The coil spring 129 is disposed in aspace between the mutually inwardly-directed frame members 135 and whentensioned by stretching urges the edge of the armature from which theears 125 project in an oblique direction toward the outlet side of theground fault circuit interrupter and away from the floor 109 of theframe 107.

As a result of the angular relationship between the axis of the spring129 and the plane of the armature 116, the force exerted by thetensioned spring 135 on the armature 116 has two vector components, oneparallel to the plane of the armature 116 and the other transverse tothat plane. The parallel vector component urges the armature and resetbutton or plunger 101 in a direction to move the reset button 101 into araised position projecting beyond the outer side of the ground faultcircuit interrupter receptacle while the transverse vector componenturges the ears 125 against the edge regions 115 or 117 of the slot 113depending upon the degree of depression of the armature 116. As thereset button 101 is pressed inwardly from the face or outlet side of theground fault circuit interrupter, the armature 116 is also forcedinwardly and each ear 125 moves along a respective edge region 115 of aslot 113 until it passes a shoulder 119 at which point it is abruptlypulled against corresponding edge region 117 of the slot 113 where it isheld by the force of the spring 135 against respective shoulder 119, theshoulder 119 serving as a latch point for the latching ears 125.

In order for the latching ears 125 to be freed from the latch points atthe shoulders 119, they must be moved in a direction transverse to theplane of the armature, that is, the armature must be pivoted about pivotears 121 in apertures 123 toward the frame floor member 109. Anelectromagnet including coil 61 wound about a cylindrical core 62 isaffixed to the frame floor member 109 by means of a rivet 141 with theaxis of the electromagnet being vertical and substantially transverse tothe variable plane of the movable armature 116. The coil 61 is normallyde-energized as previously explained with reference to FIG. 1 and whenenergized, exerts an attractive force on the armature 116 which is madeof a magnetizable metal.

The armature 116 is placed in a latched position by depressing the resetbutton 101 which thereby forces the latching ears 125 past the shoulder119 to latch adjacent side edge regions 117 under the force of thespring 135 which becomes tensioned as the armature moves inwardly. Whenthe coil 61 is energized, the armature 116 is attracted to the core 62.The armature 116 pivots on ears 121 in apertures 123 until the ears 125clear the shoulder 119 at which time the armature 116 and reset button101 move abruptly outward with the reset button 101 then projectingbeyond the face of the receptacle and the ears 125 being containedagainst respective vertical edges 143 of the slots 113.

When the armature is depressed by applying pressure to the surface 106of the reset button 101, the edge of the armature 116 between ears 125engages two outwardly extending prongs 151 (in the drawings one prong151 is hidden by the other) of a leaf spring 153. The leaf spring 153has a surface parallel and proximate to the face of the receptacle whichis substantially H-shaped with downwardly extending prongs 151comprising two spaced lower parallel legs of the H and opposite upwardlyextending legs 155 entrapped under tension behind either side of the lip159 of a support member 157 which is mounted on a vertical wall 160which is fixedly connected to the lower housing 103. The lip 159cooperates with two inward extending projections at the distal ends ofthe legs 155 of the leaf spring 153 to partially tension the leaf spring153 and keep the legs 155 trapped beneath the lip 159. Another prong 161of the leaf spring 153 projects from the crossbar of the H-shaped leafspring first towards the legs 151 and then bends under and back towardsthe legs 155 and is urged against the wall 160.

The crossbar of the H-shaped spring 153 is apertured at 163 to receiveone end of a substantially V-shaped wire spring 165. The other end ofthe spring 165 is disposed in a notch 167 formed in a projection 168extending from the middle of a rocking member 169 which supports linecontacts 11 (visible in FIGS. 3 and 5) and 17 (hidden in FIGS. 3 and 5).The rocking member 169 is substantially L-shaped at either of its ends(the face of one end being visible in FIGS. 3 and 5) and deep enough topermit mounting of two rivets 170 on opposite sides of projection 168each of which affixes one leg of a respective L-shaped conductor to aparallel face of the rocking member 169, the conductors supportingcontacts 11 and 17 on their transverse legs. When the ground faultcircuit interrupter is in a latched condition with the reset button 101depressed and ears 125 of the armature 116 entrapped at the catch points119 in the slots 113, the armature 116 exerts an inward pressure on thelegs 151 of the H-shaped spring 153 thereby tilting the attached end ofthe wire spring 165 downwardly so that its other end urges the notchedportion 167 of the rocking member 169 upwardly. The rocking member 169is urged, at all times, against a pivot member 171 which is affixed tothe wall 160 in spaced relationship to the support member 157 and whichhas a downward extending lip 172 which overhangs an upwardly extendinglip 174 on the middle portion of the L-shaped rocking member 169. Therocking member 169 is, as a result of the dispositions of the supportmember 157 and pivot member 171 confined by the tension in the spring165 between the members 169 and 171. The lip 174 of the rocking member169 intermediate the contacts 11 and 17 is shortened and abuts against acorner of the pivot member 171 to form a pivot region 173 about whichthe rocking member 169 can be rotated relative to the pivot member 171.Rotation of the rocking member 169 is limited in one direction by thesupport member 157 and in the other direction at least initially by theengagement of the respective contacts. The wire spring 165 urges therocking member 169 towards the pivot member 171 and, when the legs 151of the H-shaped spring 153 are depressed by the armature 116, urges therocking member 169 to pivot away from the supporting member 157 therebycausing the contacts 11 and 17 to rotate toward load contacts 31 and 29(behind 31 in FIGS. 3 and 5) to complete a circuit between the line andload terminals of the ground fault circuit interrupter.

A spring loaded plunger 181 is disposed beneath one of the legs 155 ofthe H-shaped spring 153 so that when the legs 151 of the H-shaped spring153 are under pressure of the armature 116, the plunger 181 is depressedthereby causing a resilient conductor 183 which is connected to thecontact 31 to engage contact 63 thereby closing the auxiliary contactswhich connect the relay coil 61 to the load side of the phase wire 25.

Upon occurrence of a ground fault, the coil 61 is energized aspreviously described and the armature 116 is attracted to it. Thearmature begins to rotate until the ears 125 clear the latch point onthe shoulder 119 in the frame 107 at which time the armature 116 beginsto move longitudinally toward the face of the receptable urged by thespring 135, as shown in FIG. 4. Linear motion of the armature continuesuntil the ears 125 engage edge 143 of the slot 113 in the frame 107 asshown in FIG. 5.

With the armature 116 in a raised position, pressure on the arms 151 isreduced and the H-shaped spring 153 rotates upwardly about a fulcrumformed between the legs 155 of the H-shaped spring and the supportmember 157. Pressure on the auxiliary contact button 181 is thenrelieved and the spring loaded plunger 181 moves longitudinallypermitting the biased contact arm 183 to separate from the auxiliarycontact 63 thereby breaking the electrical connection between auxiliarycontacts 31 and 63.

As the H-shaped spring 153 rotates upwardly the wire spring 165 istilted so that the notched end portion of the rocking member 169 isurged inwardly thereby pivoting about the pivot member 171 and causingthe main contacts 11 and 31, and 17 and 29 to open. Switches 13 and 19which were referred to in connection with the discussion of theschematic diagram of FIG. 1 comprise the rocking member 169, pivotmember 171, support member 157, springs 153, 165 and 135, armature 116,frame 107 and reset button 101. Switch 65 comprises the foregoing incombination with plunger 181 and flexible bias contact arm 183.

It will be appreciated that the previously described circuitinterrupting mechanism although illustrated in the environment of aground fault circuit interrupter can be utilized with or without thesensors 33 and 91 in other environments including circuit breakersintended to trip upon circuit overload. It will further be appreciatedthat the foregoing description is of one preferred embodiment of theinvention which can be applied in other embodiments and which is to belimited only by the following claims.

What is claimed is:
 1. A method of interrupting an electrical connectionbetween input conductors and output conductors of a circuit in responseto a current imbalance across phase and neutral power lines of saidcircuit, comprising the steps of:(a) sensing said current imbalance bymeans of a differential transformer and generating a voltage pulse inresponse thereto; (b) integrating said voltage pulse by means of anintegrating circuit directly connected to said differential transformer;(c) causing an element of said circuit to conduct current and open aswitch in response to an integrated voltage pulse of a predeterminedmagnitude and duration, and (d) causing an interrupting of saidelectrical connection in response to the opening of said switch.
 2. Themethod of claim 1 further comprising testing said circuit by closing atest switch connected between said neutral power line of said circuitand a phase wire connected to the opposite side of a differentialtransformer as said neutral power line to thereby create a currentimbalance between said neutral line and said phase wire.
 3. The methodof claim 2, further comprising connecting a visual indicating means inparallel with said output conductor of said circuit, said visualindicating means giving positive indication if power from a power lineis applied to said phase and neutral power lines of said circuit andeither said electrical connection is closed and/or said outputconductors are inadvertently directly connected to said power line. 4.The method of claim 3 wherein said visual indicating means is a lightemitting diode.
 5. The method of claim 2 further comprising detecting aneutral to ground fault in said circuit by connecting a primary windingof a power transformer across said output conductors through saidelectrical connection, and connecting a secondary winding of said powertransformer to said element, causing said element to conduct current andto open said switch and said electrical connection.
 6. The method ofclaim 1 further comprising causing said element of said circuit to stopconducting current when said switch opens.